1. Field of the Invention
The present invention relates to a thermal transfer sheet applicable to a thermal transfer printer using a heating means such as a thermal head, a laser or the like, particularly to a thermal transfer sheet capable of printing even a rough paper having a low smoothness with an image excellent in a sharpness eliminating voids and nicks and a wear resistance.
2. Description of the Related Art
A heat meltable type thermal transfer sheet is generally constituted of at least a substrate, a heat fusible ink layer and a heat resistance layer, in which a heat resistance layer is formed on one side of a substrate, and the mono-layered or multi-layered ink layer is formed on the other side of the substrate. Among methods using a thermal head with such a thermal transfer sheet, relatively major is a cold-peeled printing method, in which the ink layer is heated by the thermal head, the softened or molten ink is fixed on a receiving sheet, the fixed ink is left to be cooled, and thereafter the cooled thermal transfer sheet is peeled off the receiving sheet.
The heat meltable type thermal transfer method is desired to enable a good printing to a receiving sheet having a low smoothness. But when the rough receiving sheet is subjected to the cold-peeled printing method, it is difficult that concave portions of the rough surface is completely filled with the molten ink, thereby causing voids and nicks of the printed image. In order to attain this desire, there has been known a printing technique using one kind of the thermal heads, called as an xe2x80x9cedge headxe2x80x9d or xe2x80x9cnear-edge headxe2x80x9d, in which heating elements are disposed at an end portion or vicinity thereof on a surface of a support member of the thermal head.
Such an edge or near-edge head is applied to a hot-peeled printing method, in which the ink layer is heated by the thermal head, and simultaneously peeled off the receiving sheet while the ink layer is still soften or molten. A known thermal transfer sheet applicable to the hot-peeled printing method has a multi-layered ink layer which is constitute a peeling layer, a colorant layer and an adhesive layer laminated in this order.
When the hot-peeled printing method is carried out with the combination use of the above descried thermal transfer sheet and the edge or near-edge head, the thermal transfer sheet is peeled off the receiving sheet in a condition where the peeling layer and the adhesive layer are softened or molten respectively. Taking this mechanism into account, the thermal transfer sheet is designed so that a melt viscosity of the adhesive layer is made larger than that of the peeling layer, and thus peeling of the thermal transfer sheet is caused by cohesive failure of the peeling layer. The hot-peeled printing method can prevent voids and nicks of the printed image at the use of the receiving sheet having a low smoothness, even when the ink is not sufficiently infiltrated into the concave portion of the receiving sheet, because the peeling layer can cause cohesive failure as far as it has a sufficiently low melt viscosity to separate the colorant layer of a heated area from the substrate, and the separated colorant layer can be transferred on the concave portion to make a bridging structure over the concave portion.
However, the ink transferred in the bridging structure is in contact with only convex points of the rough surface of the receiving sheet, and it is not substantially fixed to the receiving sheet. Accordingly, a printed product obtained from the conventional hot-peeled printing method has been insufficient in a wear resistance, a scratch resistance and a plasticizer resistance of the printed image.
This invention has been achieved under these circumstances, and the object of this invention is to provide a thermal transfer sheet which can print an image having a quality appearance with no void and nick and an excellent wear resistance on a receiving sheet having a low smoothness, in particular a rough paper.
To attain this object, a thermal transfer sheet of the present invention comprises a substrate, a peeling layer and a colorant layer, the peeling layer and the colorant layer being disposed on one side of the substrate in this order when viewing from the substrate,
wherein an applied amount of the peeling layer is in a range of 1.5 g/m2 to 5.0 g/m2, an applied amount of the colorant layer is not more than half the applied amount of the peeling layer, the peeling layer has a melt viscosity in a range of 10 cP to 300 cP at 100xc2x0 C., and the colorant layer contains at least one thermoplastic resin selected from the group consisting of ethylene-acrylic acid copolymers, ionomer, styrene-butadiene rubbers, and nitrile-butadiene rubbers at a proportion in a range of 10 to 60% by weight.
In the thermal transfer sheet of the present invention, the colorant layer is laid on one surface of the substrate via at least the peeling layer, and an applied amount of the peeling layer is controlled within a range of 1.5 to 5.0 g/m2, and further an applied amount of the colorant layer is controlled to half the applied amount of the peeling layer or less. When such a thermal transfer sheet as designed above is used to print on the rough paper or another rough receiving sheet, it is assumed that the peeling layer can push the colorant layer into the concave portions of the rough surface of the rough paper, and the peeling layer can also infiltrate into the rough paper through the colorant layer, thereby improving transferability.
The peeling layer causes cohesive failure itself at a printing process to be peeled off the substrate, and it is preferable to control a melt viscosity of the peeling layer so as to show 10 to 300 cP at 100xc2x0 C.
Besides, in order to ensure a wear resistance and a scratch resistance for the image printed on the receiving sheet having a low smoothness such as the rough paper, it is preferable that the colorant layer is imparted with a good adhesive strength to the receiving sheet and a proper strength in film forming. When the colorant layer contains one or more kinds of thermoplastic resins selected among ethylene-acrylic acid copolymers (EAA), ionomer, styrene-butadiene rubbers (SBR), and nitrile-butadiene rubbers (NBR) at a proportion in a range of 10 to 60% by weight, the adhesive strength and the film strength of the colorant layer can be improved, and in addition the colorant layer can be also improved in fitness to the concave portion of the rough surface.
It is more preferable to control the melt viscosity of the peeling layer so as to show 20 to 150 cP at 100xc2x0 C.
The colorant layer becomes particularly preferable when it contains at least one of the ethylene-acrylic acid copolymers and at least one of the styrene-butadiene rubbers at a total proportion in a range of 10 to 60% by weight. Among the styrene-butadiene rubbers, carboxy-modified styrene-butadiene rubber is particularly preferable, and more advantageous effect obtained by incorporating the carboxy-modified SBR into the colorant layer at a proportion in a range of 20 to 40% by weight.